Papillary traces are one of the most effective tools in identifying individuals in criminal investigations. Papillary traces may be classified in three categories: visible traces, impressed traces, and latent traces. In the analysis of a crime scene, the investigators are looking to obtain portable and durable copies of the papillary traces, by making a photograph of them, for example. The visible traces can be photographed directly, and the impressed traces can also be photographed under certain lighting conditions.
The latent traces are the most numerous and the most difficult to lift. This is because they have a low or even zero visibility in direct light and must therefore be made to show up using a variety of methods, by increasing the contrast between the traces and the substrate. The traces are produced by the deposition of a complex mixture of natural secretions from three types of glands: the ecrine and apocrine sweat glands, and the sebaceous glands. The sweat glands produce perspiration and are distributed over the entire body. The palmar and plantar surfaces of the skin are exclusively linked to ecrine glands, whose secretions are evacuated via pores situated on the summit of the papillary ridges. These secretions are composed of inorganic substances such as the following: water (98%), chlorides, sulphates, phosphates, ammonia and metal ions. They also include organic substances such as the following: amino acids, uric acid, lactic acid, urea, sugars, creatinine and choline. The latent traces are therefore originally produced by the deposition of sweat from ecrine glands. They are often contaminated with fats originating from the sebaceous glands: the act of touching the face carries a mixture of sweat and fats onto the surface of the hands.
The techniques of visualization are therefore directed at the constituents produced by the ecrine glands (especially water, amino acids and chlorides), but also to the fatty substances present as a result of contamination.
The use of cyanoacrylates for the visualization of papillary traces is a method universally employed by the forces of order. The technique is simple to execute and particularly effective. Cyanoacrylates are a class of molecules commonly used as glues and often referred to as SUPERGLUE® or CYANOLITE®. In this method, the cyanoacrylates enter into a nucleophilic polymerization reaction. Any site or species sufficiently rich in electrons may act as the initiating nucleophile of the polymerization reaction. More particularly, water or else —COOH functions of fatty acids may act in this way. This polymerization reaction allows the cyanoacrylate in particular to reveal the latent traces, primarily the papillary prints. This reaction takes place by sublimation or fumigation of the cyanoacrylate. The molecules of vapours originating from cyanoacrylate combine with the sudoral deposits to form a chemical chain (polymerization reaction), which colours the ridges of the traces white and at the same time fixes them. This technique is employed effectively to fix the traces present on the majority of both non-porous and semi-porous substrates, and more particularly on flexible or rigid plastics, metals, glass, and varnished, painted or glazed wood, etc.
The main limitation on the technique of cyanoacrylate fumigation is linked to the white colour of the visualization. The contrast between the visualized trace and the substrate on which this trace is found is not always sufficient. In order to reinforce the contrast, the usual solution is to use low-angle lighting, or to apply a fingerprinting powder or else a stain after visualization. A combination of a number of techniques is therefore necessary to obtain a satisfactory result. The use of stains, in the form of powders or solutions, has the advantage of being adaptable to the particular configurations: the nature of the stain and its colour may be selected from a fairly broad range, in order to allow optimum staining of the trace and an effective contrast. Unfortunately, some of these methods degrade DNA, thereby preventing their use before biological material is sought, and do not always offer a satisfactory solution (technical limitation, time, cost, etc.).
Accordingly, the solutions commonly recommended are not all compatible with the subsequent examinations to which the trace or the substrate on which it is deposited may be subjected, and none of them provides an effective solution to the problem of trace resolution in all of the practical cases encountered, especially irrespective of the substrate on which the trace is deposited or of the subsequent examinations.